Treatment of reclaim water for use in metals recovery

ABSTRACT

A process for improving the recovery of gold from gold-bearing solutions formed from carbonaceous ore using a cyanide lixiviant, the process comprising treatment of tailings pond water with H 2 SO 5  before introduction into a gold-recovery system such that the water has a total concentration of CN −  and SCN −  reduced by at least 30%.

FIELD OF THE INVENTION

This invention generally relates to gold recovery and the treatment and use of reclaim water in gold-bearing recovery circuits having carbon-in-leach or carbon-in-pulp operations.

BACKGROUND OF THE INVENTION

Gold is frequently recovered from gold-bearing materials by leaching the gold into a leachate solution, for example, by contacting the gold-bearing material with a lixiviant. One method conventionally employed for recovering gold from gold-bearing materials is leaching with cyanide and adsorption of gold-cyanide onto activated carbon.

In modern cyanidation circuits, the dissolved gold is typically adsorbed onto particles of activated carbon, either during a cyanide leach itself by carbon-in-leach (CIL) or following the leach by carbon-in-pulp (CIP). An alternate method of recovering gold from cyanide leach solutions is through zinc cementation and variations of the Merrill-Crowe process.

While cyanide leaching has been proven over decades as effective and environmentally safe when practiced under accepted handling and destruction procedures, millions of dollars of gold is still lost to “preg robbing.” In carbonaceous ores, preg robbing occurs as active carbon indigenous to the ore has the ability to rob gold from the cyanide bearing leach solution, reducing recovery.

Pressure oxidation, as described by Thomas et al. (U.S. Pat. Nos. 5,074,477 and 5,785,736) and incorporated in their entirety herein by reference, can partially deactivate the indigenous carbon. Much of the carbonaceous ore is unaffected, however, allowing it to adsorb gold from cyanide solutions. Also, CIL has been successful for mildly preg robbing ores, as the activated carbon that is added to the slurry possesses adsorption kinetic characteristics superior to those of the indigenous carbon, allowing the gold to load onto the added carbon as soon as it is leached and before it can load onto the carbon in the ore. Other methods of reducing the preg robbing effect include destroying the preg robbing carbon with a roaster or with chlorine, using an alternative lixiviant such as thiosulfate, using ion exchange resins to load the gold, and oxidizing extremely fine ore under extreme oxidizing conditions in a low chloride environment.

To initially form the gold-bearing slurry, the gold-bearing material is crushed and wet milled prior to a liquid/solid separation stage using liquid recovered from the system. This liquid typically comprises so-called “make-up” or “recovery” water and is also utilized, e.g., in post-autoclaving cooling and neutralization. The liquid, which is held in a tailings pond until such use, further comprises a quantity of cyanide (CN⁻) and thiocyanate (SCN⁻) ions. As used in the art and throughout this application, the liquid going into, held in, and coming from the tailings pond is typically called simply “water” even though it is actually water with a quantity of CN⁻, SCN⁻, and other compounds.

When CN⁻ or SCN⁻ is present in the gold-bearing slurry, it can form soluble Au complexes, which are then unintentionally loaded onto indigenous during the pre-cyanidation treatment. Accordingly, there are gold losses due to the presence of CN⁻ or SCN⁻ in the gold-recovery operation prior to the gold-recovery stage itself.

SUMMARY OF THE INVENTION

Among the several objects of the present invention, therefore, is the provision of a process for recovering gold from comminuted ores, concentrates, or other feed materials from which gold has been leached wherein the concentration of CN— and SCN⁻ in the water taken from the tailings pond is reduced prior to introduction into the gold-recovery process, thereby allowing a greater quantity of gold to remain in the solution and, ultimately, to be recovered.

Briefly, therefore, the invention is directed to a process for recovering gold from carbonaceous ore in a cyanide-based gold recovery operation, the process comprising directing tailings from the cyanide-based gold-recovery operation to a reservoir, the tailings comprising CN⁻ and SCN⁻; holding the tailings in the reservoir; removing an initial water effluent comprising an initial total concentration of CN⁻ and SCN⁻ from the reservoir; treating the initial water effluent to oxidize CN⁻ and SCN⁻ to yield a treated water effluent having a treated total concentration of CN⁻ and SCN⁻ which is at least 30% below said initial total concentration of CN⁻ and SCN⁻ in the initial water effluent; and introducing said treated water effluent into a gold-recovery operation to recover gold from carbonaceous ore with preg robbing tendencies.

In another aspect the invention is directed to a process for treating water for use in a gold-recovery system, wherein the water has an initial total concentration of CN⁻ and SCN⁻, the process comprising adding H₂SO₅ to the water to yield treated water having a total treated concentration of CN⁻ and SCN⁻ which is at least 30% less than said initial total concentration of CN⁻ and SCN⁻, wherein the initial total concentration of CN⁻ and SCN⁻ is above about 20 ppm.

These and other objects, features, and advantages of the invention will become apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a flow diagram illustrating the overall process for one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with this invention, water introduced to a cyanide leaching gold recovery process is treated so as to decrease the concentration of CN⁻ and SCN⁻, thereby diminishing the preg robbing phenomena and improving the gold-recovery yield from carbonaceous ore. One preferred gold recovery process to which the invention is applicable is shown in FIG. 1.

According to such a process, ore is crushed during a grinding step performed by a milling process, such as one or more semi-autogenous (SAG) mill/ball mill circuits or other grinding circuits, after which the ground ore slurry is screened for trash or tramp material. Lime is optionally added during the grinding step to adjust the working pH of the slurry as desired, commonly, for example, between about 9 and about 9.5. The ground ore slurry is thickened, if necessary, by removal of excess water in a solid-liquid separation operation. The thickened slurry may comprise any suitable or desired portion of solids, with between about 50% and about 70% or between about 55% and about 60% as common slurry compositions. The excess water is stored and reused in the grinding step.

The slurry is directed to acidulation tanks where a sufficient amount of sulfuric acid is added to dissolve carbonate materials. This process yields liberated CO₂, which is subsequently volatilized as much as possible. Thereafter, the slurry is pumped to the acid pressure oxidation vessels to break down the sulfides and liberate the encapsulated gold. There, oxygen and steam are introduced to the pressure vessel to attain the necessary temperature and desired degree of sulfide oxidation. Energy from the exothermic pressure oxidation is recovered by heat exchange between the oxidized slurry and acidulated feed.

After it is partially cooled, the oxidized slurry is further cooled and passed directly to a neutralization operation. Here, a base, such as milk of lime, is added to adjust the pH to allow for subsequent cyanide leaching. Typically, the pH value is adjusted to between about 10 and about 10.5. Gold is recovered from the neutralized, oxidized slurry by any known means such as, for example, CIL cyanidation in a continuous countercurrent system. The barren slurry, known as tailings, is directed to and held in a reservoir (tailings pond).

The tailings typically comprise, e.g., recovery water, residual lixiviant, neutralizing agent, and comminuted ore. When cyanide is used as the lixiviant, untreated tailings comprises residual CN⁻ and SCN⁻ at a concentration between about 15 ppm and about 40 ppm. According to common practice in the art, the tailings is treated to reduce the CN⁻ concentration. This treatment typically occurs either before or shortly after entry into the tailings pond. Typically, oxidizers such as hydrogen peroxide or Caro's acid (peroxymonosulfuric acid) can be used to reduce the concentration of free CN⁻, weakly acid dissociable (WAD) cyanides, and highly complexed cyanides. While these forms of cyanide are reduced, they are not completely eliminated in the traditional practice because any amount remaining in the tailings will reduce the amount of CN⁻ that must be added after water from the tailings pond is reintroduced into the gold-recovery system.

In accord with this invention, after a period of time during which the solids settle to the bottom of the tailings pond, a water effluent is removed from the tailings pond and is reintroduced into the gold-recovery process. Typically, this water comprises some residual lixiviant and neutralizing agent. In a typical gold-recovery system wherein cyanide is used as the lixiviant, this water comprises, for example, CN—, usually as a WAD cyanide, in a concentration between about 5 ppm and about 20 ppm and SCN⁻ in a concentration of between about 10 ppm and about 30 ppm. The total concentration of CN⁻ and SCN⁻ is more than about 20 ppm, typically more than about 25 ppm, and often more than 30 ppm or 40 ppm.

In accord with this invention, the water effluent is treated with Caro's acid (peroxymonosulfuric acid, H₂SO₅) to reduce the concentration of both CN⁻ and SCN⁻ by at least 30%. In certain embodiments it is reduced by more than 40 or 50%, such as by more than 70%. Generally, Caro's acid is added to the water in an amount sufficient to reduce the total concentration of CN⁻ and SCN⁻. For example, Caro's acid is added to the water in an amount sufficient to reduce the total concentration of CN⁻ and SCN⁻ to below about 20 ppm, such as to below about 10 ppm. In one preferred embodiment, the total concentration of CN⁻ and SCN⁻ is below about 5 ppm. In the most preferred embodiment, there is no detectable CN⁻ or SCN⁻ present in the water reintroduced into the gold-recovery system.

In accordance with this invention, the amount of Caro's acid necessary to obtain the desired total concentration of CN⁻ and SCN⁻ can be determined by calculating the stoichiometric requirement of the amount of Caro's acid necessary to react with the concentration of CN⁻ and SCN⁻ in the untreated tailings. In general, the amount of Caro's acid added to the untreated tailings is greater than about 100% of this stoichiometric requirement. For example, the amount of Caro's acid added is typically greater than about 150%, 175%, or 200% of the stoichiometric requirement. In one preferred embodiment, the amount of Caro's acid added to the recovery water is greater than about 125% of the stoichiometric requirement.

In accordance with this invention, the Caro's acid may be added to the water at any point after it leaves the tailings pond and before introduction into the gold-recovery process. In one preferred embodiment, the Caro's acid is added to the water after reentry into one of the gold-recovery system components, e.g., a holding tank, directly before the water is introduced to a particular process step. Regardless of when the Caro's acid is added to the tailings, it may be added continuously, at regular intervals, or when necessary as determined by monitoring the CN⁻ and SCN⁻ concentration. Furthermore, the treated water may be introduced at any process step utilizing water, e.g., to grinding, lime slakers, exhaust scrubbers, and pump gland seals.

Upon introduction to the water, Caro's acid reacts very quickly, first with CN⁻ and then with SCN⁻, to reduce their concentration. The Caro's acid reacts with the CN⁻ according to the following reaction: H₂SO₅+CN⁻->H₂SO₄+OCN⁻ With thiocyanate, the Caro's acid oxidizes sulfur and liberates free cyanide according to the following reaction: 3 H₂SO₅+SCN⁻⁻+H₂O->OCN⁻+4 H₂SO₄ Then, the Caro's acid oxidizes the liberated free cyanide to obtain the following overall reaction: 4 H₂SO₅+SCN⁻⁻+H₂O->OCN⁻+5 H₂SO₄ Unlike many other commonly used acids, Caro's acid is strong enough to oxidize the SCN⁻ after an amount has reacted with the CN⁻.

Without being bound to a particular theory, the present invention improves the final yield of a gold-recovery process by limiting the preg robbing effect of CN⁻ and SCN⁻ exposed to the ore prior to the autoclave process. By comparison, conventional gold-recovery processes comprise treatment of the tailings to control only the concentration of the CN⁻. In such processes, control of or even complete elimination of CN⁻ has not proved successful in reducing the preg robbing phenomena. In accord with the present invention, successfully improving the yield of the gold recovery process depends on reducing the recovery water's SCN⁻ concentration in addition to reducing the CN⁻ concentration. Under acidic conditions, SCN⁻ can leach gold and contribute to the preg robbing effect during the gold-recovery process. Furthermore, when SCN⁻ oxidizes under conditions such as those surrounding the autoclave, it can form free CN⁻, which will leach gold and cause preg robbing. Furthermore, in trying to limit the SCN⁻ concentration, oxidation is difficult, such that most oxidizers are not powerful enough to reduce the SCN⁻ concentration. But by introducing Caro's acid, a strong oxidizer, just before reintroduction of the recovery water into the gold-recovery system, the SCN⁻ concentration is efficiently controlled. Also, merely modifying the known practice by adding Caro's acid to the tailings before entry into the tailings pond to reduce the CN⁻ and SCN⁻ concentration is nearly impossible and, therefore, commercially impractical. This is because (1) the tailings comprises several other constituents that would react with the Caro's acid, such as the comminuted ore solids, reducing the Caro's acid effectiveness in oxidizing the SCN⁻, and (2) SCN⁻ is difficult to oxidize, such that an excessive amount of Caro's acid would be required in view of the large volume of the pond.

In general, Caro's acid is added to the tailings in an amount sufficient to sufficiently oxidize CN⁻ and SCN⁻, to decrease preg robbing tendencies, and thereby improve the overall recovery of gold from the gold-recovery process by at least about 1% as compared to a gold-recovery process using tailings water treated only prior to or in the tailings pond for environmental reasons (conventional process). For example, the overall recovery of gold from the gold-recovery process is improved by at least about 2% as compared to a conventional process. In one preferred embodiment, the overall recovery of gold from the gold-recovery process is improved by at least about 3% as compared to a conventional process. With ores that have greater preg robbing tendencies, the improvement is typically greater.

In addition to the Caro's acid, a base may optionally be added to the water from the tailings pond to adjust the pH of the water. One base commonly used is milk of lime. The base is added in an amount sufficient to bring the pH of the water between about 8 and about 11 prior to reintroduction into the gold-recovery process. For example, the pH of the water is between about 9 and about 10, such as about 9.5. The precise amount of base necessary will depend on the strength thereof.

In accord with a further optional step of the invention, the treated recovery water may be clarified or strained to remove any solids therefrom before reintroduction. These solids are removed to prevent problems such as plugging or scaling of processing equipment at the processing steps where the water is reintroduced. The clarification or straining can be accomplished by any means known in the art, such as, for example, a clarifier, filter, screen, or cyclone.

EXAMPLES

Further illustration of the invention is provided by the following examples:

Example 1

An experiment was conducted using an semi-continuous autoclave (SCAC) oxidation procedure, rather than a batch test.

SCAC tests were performed in a semi-continuous 2-liter agitated pressure vessel at about 225° C. under about 0.69 MPa oxygen overpressure with a pulp density of about 40% solids. Approximately every 6 minutes, a quarter of the slurry volume was withdrawn from the vessel and replaced with the same volume of fresh, untreated slurry. To ensure that the process had stabilized, the first eight slurry withdrawals, equivalent to two autoclave volumes, were discarded. Thereafter, each withdrawal from the autoclave was collected and composited for the bottle roll test. The nominal retention time for the SCAC test is about 24 minutes. The average residence time is about 23.2 minutes after three volume turnovers and about 23.8 minutes after four volume turnovers.

Example 2

Ore samples that had been oxidized using the SCAC procedure described in Example 1 were bottle-roll leached with cyanide for about 16 hours in the presence of approximately 12 g of virgin activated carbon, with a pulp density of about 40% solids. The slurry was then subjected to the gold reclaim process in three settings: using untreated recovery water, using recovery water treated with Caro's acid, and using DI water.

For the treated recovery water trial, Caro's acid and milk of lime were added simultaneously to the reclaim solution, such that the pH was maintained at about 9.5 to 10. Caro's acid was added in an amount equivalent to about 125% of the stoichiometric requirement. After approximately 20 minutes of agitation, the Caro's acid was able to reduce the concentrations of SCN⁻ and WAD cyanide to 0.13 and 0.8 ppm, respectively.

The data below shows the results of gold recovery processes for three different carbonaceous ore samples. The process using DI water is the control for these trials because of the absence of both CN— and SCN⁻. In each trial, the treated recovery water yielded better gold recovery than the untreated recovery water, with the improvement ranging between about 1.1% to about 2.9%. Caro's acid Head Tails % Stoich g/t Au g/t Au Au Rec %  0 5.97 1.06 82.2 125 5.97 0.994 83.3 DI 5.97 1.06 82.2  0 4.70 0.789 83.2 125 4.70 0.651 86.1 DI 4.70 0.651 86.1  0 6.51 1.27 80.5 125 6.51 1.13 82.6 DI 6.51 1.10 83.2

When introducing elements of the present invention or the preferred embodiment(s) thereof, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.

The foregoing relates to a limited number of embodiments that have been provided for illustration purposes only. It is intended that the scope of invention is defined by the appended claims and there are modifications of the above embodiments that do not depart from the scope of the invention. 

1. A process for recovering gold from carbonaceous ore in a cyanide-based gold recovery operation, the process comprising: removing tailings from the cyanide-based gold-recovery operation, the tailings comprising CN⁻ and SCN⁻; holding the tailings in a reservoir; removing an initial water effluent comprising an initial total concentration of CN⁻ and SCN⁻ from the reservoir; treating the initial water effluent to oxidize CN⁻ and SCN⁻ to yield a treated water effluent having a treated total concentration of CN⁻ and SCN⁻ which is at least 30% below said initial total concentration of CN⁻ and SCN⁻ in the initial water effluent; and introducing said treated water effluent into a gold-recovery operation to recover gold from carbonaceous ore with preg robbing tendencies.
 2. The process of claim 1 wherein the treated total concentration of CN⁻ and SCN⁻ is below about 10 ppm.
 3. The process of claim 1 wherein the treated total concentration of CN⁻ and SCN⁻ is below about 5 ppm.
 4. The process of claim 1 wherein the treated water effluent has no detectable treated total concentration of CN⁻ and SCN⁻.
 5. The process of claim 1 wherein said treating the initial water effluent to oxidize CN⁻ and SCN⁻ comprises treating the initial water effluent with H₂SO₅.
 6. The process of claim 5 wherein the initial water effluent is treated with at least about 100% of the stoichiometric requirement of H₂SO₅ necessary to react with the total initial concentration of CN⁻ and SCN⁻.
 7. The process of claim 5 wherein the initial water effluent is treated with at least about 125% of the stoichiometric requirement of H₂SO₅ necessary to react with the total initial concentration of CN⁻ and SCN⁻.
 8. The process of claim 5 wherein the initial water effluent is treated with at least about 150% of the stoichiometric requirement of H₂SO₅ to react with the total initial concentration of CN⁻ and SCN⁻.
 9. The process of claim 5 wherein the initial water effluent is also treated with a base in an amount such that the pH of the initial water effluent is between about 8 and about
 11. 10. The process of claim 5 wherein the initial water effluent is also treated with a base in an amount such that the pH of the initial water effluent is between about 9 and about
 10. 11. The process of claim 5 wherein the initial water effluent is also treated with a base in an amount such that the pH of the initial water effluent is about 9.5.
 12. The process of claim 1 wherein the water is introduced to the gold-recovery system at a processing step selected from the group consisting of grinding, lime slakers, exhaust scrubbers, pump gland seals, and any combination thereof.
 13. The process of claim 2 wherein the initial total concentration of CN⁻ and SCN⁻ is at least about 30 ppm.
 14. A process for treating water for use in a gold-recovery system, wherein the water has an initial total concentration of CN⁻ and SCN⁻, the process comprising: adding H₂SO₅ to the water to yield treated water having a total treated concentration of CN⁻ and SCN⁻ which is at least 30% less than said initial total concentration of CN⁻ and SCN⁻, wherein the initial total concentration of CN⁻ and SCN⁻ is above about 30 ppm and the treated total concentration of CN⁻ and SCN⁻ is below about 20 ppm.
 15. The process of claim 14 wherein the water has a total concentration of CN⁻ and SCN⁻ below about 10 ppm.
 16. The process of claim 14 wherein the treated water has no detectable treated total concentration of CN⁻ and SCN⁻.
 17. The process of claim 14 wherein the water having the initial total concentration of CN⁻ and SCN⁻ is reclaim water from a gold-recovery tailings pond.
 18. The process of claim 14 wherein the H₂SO₅ is added to the water in an amount greater than about 100% of the stoichiometric requirement of H₂SO₅ necessary to react with the total initial concentration of CN⁻ and SCN⁻.
 19. The process of claim 14 wherein the water further comprises a base in an amount such that the pH of the water is between about 8 and about
 11. 20. The process of claim 14 wherein the water is introduced to the gold-recovery system at a processing step selected from the group consisting of grinding, lime slakers, exhaust scrubbers, pump gland seals, and any combination thereof. 